Syrian rebels on Friday said they launched a major offensive to break a siege by government forces in opposition-held eastern Aleppo.

The Islamic Front rebel coalition said the ultraconservative Ahrar al-Sham group launched rockets at a military airport in the eastern part of the city and destroyed a government position in the west, the Associated Press reported.

The Syrian Observatory for Human Rights, a London-based monitoring group, said rebels detonated two car bombs targeting government positions in southwestern Aleppo and attacked with more than 150 rockets.

The developments come after Syrian state media reported shelling by rebels killed several children at a school in government-held western Aleppo on Thursday.

Airstrikes on another school in the village of Haas in rebel-held Idlib on Wednesday killed at least 15 children, Reuters reported. Western countries blamed the Syrian military and the Russian air force, but Moscow denied any involvement.

Earlier this week, Russia said there had been no airstrikes over Aleppo for a week, a claim disputed by local residents who told CNN that areas in the eastern part of the city were bombed.